By analyzing the state equations of unified power flow controller（UPFC） stabilizer， the influence of UPFC stabilizer on electromagnetic torque of synchronous generator is presented. Synchronizing torque and damping torque of synchronous generators are two main factors in influencing the stability of power system. In view of the possibility of worsening system synchronizing torque while improving the system damping torque by UPFC stabilizer， the parameters of UPFC stabilizer are optimized by using maximum torque area. Larger torque area could maintain enough synchronizing torque and damping torque. The optimized UPFC stabilizer can improve the synchronizing torque as well as damping torque， and therefore， improve the stability significantly. The EMTDC simulation results for an example demonstrate the efficiency of the method proposed.